Add support for dotenv file

[pior]

Why

Add a limited support for dotenv file.

• A envfile task must be declared
• On entering the project, the .env file is read, every var defined are set in the environment
• Thanks to the auto-unload, the variables added/changed are reverted when leaving the project

Gotchas:

• the envfile is loaded only once, when entering the project
  • if the file is changed, the environment is not refreshed accordingly
• if the task is removed, the variables are not unloaded (until leaving the project)

Related to #149

How